I'm not a patent lawyer but it would seem difficult to patent a magazine unless it contained some particular improvement. The whole point of patent law is to protect a new innovation so that inventors have an incentive to make money off their invention. Just because a magazine only fits your gun does not make it innovative. When I think about innovative gun ideas, I think about things like the Beretta Cougar rotating barrel. Assuming that was a new innovation, it could be patented with a utility patent. There are, of course, design patents but those need to be "ornamental" in nature. Would the shape of a magazine fit under the classification of a design patent? Maybe, but why waste your time patenting them? I doubt Beretta made much money selling Cougar magazines (but AR15 magazines- which would be way off patent- might make some money these days).